Syringe pumps provide a mechanism for controlling the motion of the piston within the barrel and therefore the displacement of fluid in the syringe. The syringe used by the pump may be an integral part of the pump itself, or can commonly be a disposable part that can be used and replaced, and may be a syringe that can also be used manually. A syringe pump theoretically allows for a straightforward determination of the amount of fluid pumped, based on the distance moved by the piston within the barrel.
Conventional syringes and syringe pumps operate by the motion of a plunger displacing fluid within a barrel. When the plunger is advanced from an end within the barrel, fluid in the barrel is forced out the other end. On the other hand, when the plunger is drawn out of the barrel through one end, fluid is drawn into the barrel through the other end. To allow the pumping operation to occur properly, the plunger is sealed against the barrel, so that fluid cannot pass around the plunger for example.
The syringe pump can include an actuator attached to the end of the piston plunger in order to push and/or pull the plunger in/out of the syringe barrel. In such cases, it is necessary for the actuator to move with the piston, and therefore space has to be provided for this. Further, a driving mechanism for moving the actuator needs to be provided. As a result, the syringe pump often requires much more space than the syringe which it drives. For example, the overall length/height of a syringe pump can include the length of the syringe, the length of the actuator and the length of the driving mechanism.
Other syringe pumps do not directly connect an actuator to the syringe plunger. Instead, the actuator may be driven to contact and push the end of a syringe plunger without becoming connected to the syringe. This allows for the syringe to be emptied by the pump, but not filled. In such cases, the syringes may be replaced when emptied or manually re-filled. Another problem with such pumps is that the user cannot always be certain if the pump is dispensing fluid when the actuator is displaced, because the actuator may not be in contact with the syringe plunger when the actuator is first moved.
Conventional syringe pumps also suffer from a problem related to the seal formed by the syringe plunger with the syringe barrel.
One common method of sealing the plunger against the barrel in disposable syringes is by using an elastic seal such as an O-ring. Such an O-ring is provided around the outside of the plunger, close to the end of the plunger driving fluid within the barrel. The O-ring provides a relatively deformable surface that can thus shape itself to the barrel and form a tight seal. As such, the O-ring is positioned between the plunger and the barrel and so is in contact with the fluid in the barrel.
Another method of sealing the plunger against the barrel is for the whole plunger head to be made of an elastic material, so that the plunger head deforms as a whole to produce a seal within the barrel.
In both of these cases, the fluid in the barrel is in contact with the elastic material that is being used to create the seal. This raises possible fluid contamination/interaction issues. In fluidic and microfluidic applications, and biological fluidic applications in particular, it is undesirable to introduce materials into the system which might somehow react with the fluids or leach contaminants into the process fluids and somehow affect the experiment being performed in an unexpected and/or unquantifiable manner.
Further, the use of an elastic sealing material causes a characteristic stick/slip jump when first moving the plunger. This is caused due to high static friction between the elastic sealing material and barrel, which must be overcome to start the plunger moving. The high static friction means a correspondingly large force is required to start moving the plunger, and a characteristic jerk occurs before smooth plunger motion can be observed once enough force has been applied to the plunger to overcome the static friction. Even after the static friction has been overcome, the presence of the elastic O-ring results in a relatively large amount of dynamic friction (and hence a corresponding large required driving force) that must be overcome to keep the plunger moving. This makes fine control of the syringe difficult, especially when first beginning to move the plunger.
Further, it is typical for conventional syringes and syringe pumps to somewhat rely upon deformation of the outer barrel around the piston seal (be it an O-ring or the entire piston head) to achieve a good seal. That is, the barrel will deform slightly outwards in the region of the seal to allow the piston to be moved whilst maintaining pressure on the seal.
An alternative to the elastic material approach is to make the syringe (from metal for example) so the piston is an exact fit for the barrel. However, this approach is expensive and is therefore not well suited to the mass manufacture of disposable syringes. The use of metal may also introduce fluid contamination considerations.